Elucidation of the factors responsible for the substrate specificities of various cytochrome P-450 monooxygenase systems (P-450 systems) requires the means to detect and quantitate the different isozymes of cytochrome P-450. With this information and knowledge of the catalytic activities of the P-450 isozymes, the metabolic capacity of a given P-450 system can, to a great extent, be explained. Techniques for the detection and quantitation of P-450 isozymes have been developed and are being applied to a number of problems. However, these techniques have brought into question the substrate specificities that have been established for some of the enzymes. The reason for this is the sensitivity of the immunochemical methods. In the past, the criterium for the purity of an isozyme preparation used to determine substrate specificity was the appearance of the preparation following electrophoresis and staining for protein. If no impurities were observed, the preparation was judged to be "homogenious". With immunochemical methods we have shown that up to 50% contamination with a second P-450 isozyme cannot be detected by protein staining. Such contamination can, however, contribute significantly to the substrate specificity of the preparation. With immunochemical methods we have described a number of major differences between the monooxygenase systems of rabbit liver and lung. The major isozymes of rabbit lung (forms 2 and 5) are minor forms in the liver unless the animals have been treated with phenobarbital. Treatment of rabbits with polycyclic aromatic hydrocarbons results in increases of two isozymes in the liver but only one in the lung. These differences and others are now being investigated in a number of different species including humans. With antibodies to both rabbit and rat isozymes of cytochrome P-450, we have been able to demonstrate that all species examined exhibit similar differences between liver and lung and that all species have analogous isozymes of cytochrome P-450.